U.S. forces repeatedly have launched drone strikes and airstrikes in al-Bayda and southeastern Shabwa province, where dozens of al-Qaida and Islamic State members are thought to be based.

Yemen, an impoverished Arab country, has been gripped by one of the most active regional al-Qaida insurgencies in the Middle East.

Yemen's al-Qaida branch, Al-Qaida in the Arabian Peninsula (AQAP), emerged in January 2009 and has claimed responsibility for a number of terrorist attacks against Yemen's army and government institutions.

The AQAP- and IS-linked terrorists have taken advantage of the security vacuum to expand their influence and seize more territory in southern Yemen.

Security in Yemen has further deteriorated since 2015, when Iran-aligned Houthi forces loyal to Yemen's former president, Ali Abdullah Saleh, began clashing with President Abd Rabu Mansour Hadi's Saudi-backed government.

More than 10,000 people have been killed in ground battles and airstrikes since then, many of them civilians.